You don't have to wonder (or have that many followers) due to broken notification system. Google Hangouts will push all notifications since you were last online. So if you have an old phone that you rarely use... Strava, for example, is stupid like this as well.

They are slowed down somehow. Because even if I disconnect the Internet immediately after I remember what awaits me, it keeps bugging me for quite some time after. Associated ring/buzz is not "grouped" so it is a hellish experience (and perhaps that's the reason why it is so slow).

I dunno about android in general, but my old GS4 would probably crash and turn itself off.

Not sure about Marshmallow, but in Lollipop you get one notification card with a counter that is the actual number of notifications of that particular type/app. So you would not have a nice animation like in iOS.